Insulator strength by seat geometry

CROSS REFERENCE TO RELATED APPLICATION

p-0002This application claims the benefit of application Ser. No. 61/568,889 filed Dec. 9, 2011, the entire contents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

p-00031. Field of the Invention

p-0004This invention relates generally to spark plugs, and more particularly to insulator geometry of the spark plugs, and methods of manufacturing the same.

p-00052. Related Art

p-0006Spark plugs for use in combustion chambers of automotive or industrial engines include a center electrode and a ground electrode providing a spark gap therebetween. During operation, a spark forms across the spark gap to ignite a combustible mixture of fuel and air. An insulator surrounds and electrically isolates the central electrode, and also provides mechanical support to the central electrode. The insulator is surrounded by a metal shell which is threaded into a cylinder head of the engine. According to one spark plug design, the insulator includes a body region and a tapering nose region which are separated by an insulator seat. A gasket is compressed between insulator seat and shell to maintain the insulator in position. The preload on the gasket should be high enough to seal under all operating conditions. However, the high preload causes tensile stress around the gasket and along the insulator seat.

p-0007The insulator of the spark plug also experiences significant bending stress around the insulator seat when used in a high-output engine. These engines generate “mega-knock”or “super-knock” causing high pressure transient shock waves which create a force transverse to the insulator nose region.

SUMMARY OF THE INVENTION

p-0008One aspect of the invention provides a spark plug including an insulator geometry providing reduced tensile stress during installation and increased bending strength during use in a high-output engine. The insulator extends along a center axis and presents an insulator outer surface extending from an insulator upper end to an insulator nose end. An insulator body region extends between the insulator upper end and the insulator nose end. The insulator presents a first radius (R₁) at the insulator body region extending from the center axis to the insulator outer surface. The insulator also includes an insulator nose region between the insulator body region and the insulator nose end. The insulator presents a sixth radius (R₆) at the insulator nose region extending from the center axis to the insulator outer surface. The sixth radius is less than the first radius.

p-0009An insulator seat is disposed between the insulator body region and the insulator nose region. The insulator seat extends radially toward the center at an insulator seat angle. The insulator includes a convex first transition extending from the insulator body region to the insulator seat. The insulator presents a fifth radius (R₅) at the first transition, and the fifth radius is a spherical radius. The insulator also presents a concave second transition extending from the insulator seat to the insulator nose region. The insulator presents a second radius (R₂) extending from the center axis to a point at the intersection of the insulator outer surface of the insulator seat and the insulator outer surface of the insulator nose region adjacent the second transition. The insulator presents a fourth radius (R₄) at the second transition, and the fourth radius is a spherical radius. The insulator seat angle is from 35° to 50°, and the insulator seat angle is greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R₁−R₂)÷(R₄+R₅)].

p-0010Another aspect of the invention provides a method of forming the spark plug. The method includes selecting a value for the insulator seat angle between 35° to 50°; obtaining values for R₁, R₂, R₄, and R₅; and determining whether the selected insulator seat angle (α_i) is greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R₁−R₂)÷(R₄+R₅)].

p-0011The geometry of the insulator seat provides reduced tensile stress along and around the insulator seat during assembly of the spark plug, particularly reduced tensile stress caused by compressing the gasket between the insulator and shell. The geometry of the insulator seat also provides increased bending strength along and around the insulator seat when the spark plug is used in a high-output engine.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

<figref idrefs="DRAWINGS">FIG. 1</figref> is a cross-sectional view of a spark plug in accordance with one embodiment of the invention;

<figref idrefs="DRAWINGS">FIG. 2</figref> is an enlarged view of a portion of <figref idrefs="DRAWINGS">FIG. 1</figref> around the insulator seat;

<figref idrefs="DRAWINGS">FIG. 2A</figref> is an enlarged view of a portion of <figref idrefs="DRAWINGS">FIG. 2</figref>;

<figref idrefs="DRAWINGS">FIG. 3</figref> is an enlarged view of a portion of a spark plug according to a second embodiment of the invention;

<figref idrefs="DRAWINGS">FIG. 4</figref> is a cross-sectional view of a comparative spark plug; and

<figref idrefs="DRAWINGS">FIG. 5</figref> is a graph illustrating the bending strength of the spark plugs of <figref idrefs="DRAWINGS">FIGS. 1</figref>, <b>3</b>, and <b>4</b>.

DETAILED DESCRIPTION

p-0019One aspect of the invention provides a spark plug <b>20</b> for use in an internal combustion engine, as shown in <figref idrefs="DRAWINGS">FIG. 1</figref>. The spark plug <b>20</b> includes an insulator <b>22</b> with reduced tensile stress during assembly and increased bending strength when subjected to shock wave forces that occur due to mega-knock or super-knock in a high-output engine. The insulator <b>22</b> includes an insulator body region <b>24</b> and an insulator nose region <b>26</b> with an insulator seat <b>28</b> therebetween. The insulator <b>22</b> is designed to include an insulator seat angle α_iof 35° to 50° and an increased insulator thickness t_iin selected areas around the insulator seat <b>28</b>.

p-0020As shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, the insulator <b>22</b> of the spark plug <b>20</b> extends along a center axis A and presents an insulator outer surface <b>30</b> and an oppositely facing insulator inner surface <b>32</b> each extending longitudinally from an insulator upper end <b>34</b> to an insulator nose end <b>36</b>. The insulator inner surface <b>32</b> and the insulator outer surface <b>30</b> present an insulator thickness t_itherebetween, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The insulator inner surface <b>32</b> extends annularly around the center axis A and presents a bore. The insulator inner surface <b>32</b> presents an insulator inner diameter D₁surrounding the bore and the insulator outer surface <b>30</b> presents an insulator outer diameter D₂, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>.

p-0021In the embodiment of <figref idrefs="DRAWINGS">FIG. 1</figref>, the insulator <b>22</b> includes an insulator terminal region <b>38</b>, an insulator transition region <b>40</b>, the insulator body region <b>24</b>, and the insulator nose region <b>26</b>. The insulator terminal region <b>38</b> extends from the insulator upper end <b>34</b> toward the insulator nose end <b>36</b>. The insulator transition region <b>40</b> is disposed between the insulator terminal region <b>38</b> and the insulator body region <b>24</b>. The insulator thickness t_ivaries along the insulator transition region <b>40</b>. Along one portion of the insulator transition region <b>40</b>, the insulator thickness t_iis greater than the insulator thickness t_ialong the insulator terminal region <b>38</b>. Along another portion of the insulator transition region <b>40</b>, the insulator thickness t_iis less than the insulator thickness t_ialong the insulator terminal region <b>38</b> and decreases toward the insulator body region <b>24</b>. An insulator upper shoulder <b>42</b> extends from the insulator terminal region <b>38</b> to the insulator transition region <b>40</b>, and the insulator thickness t_ialong the insulator upper shoulder <b>42</b> increases from the insulator terminal region <b>38</b> to the insulator transition region <b>40</b>.

p-0022The insulator body region <b>24</b> is disposed between the insulator transition region <b>40</b> and the insulator nose region <b>26</b>. The insulator <b>22</b> presents a first radius R₁along the insulator body region <b>24</b> extending from the center axis A to the insulator outer surface <b>30</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The insulator thickness t_ialong the insulator body region <b>24</b> is less than the insulator thickness t_ialong the insulator terminal region <b>38</b> and less than the insulator thickness t_ialong the insulator transition region <b>40</b>. The ratio of the insulator inner diameter D₁to the insulator outer diameter D₁along the insulator body region (<b>24</b>) adjacent the insulator seat <b>28</b> is preferably from 0.12 to 0.45, and more preferably from 0.18 to 0.38. An insulator lower shoulder <b>44</b> extends from the insulator transition region <b>40</b> to the insulator body region <b>24</b>, and the insulator thickness t_ialong the insulator lower shoulder <b>44</b> decreases from the insulator transition region <b>40</b> to the insulator body region <b>24</b>.

p-0023The insulator inner surface <b>32</b> along the insulator body region <b>24</b> presents an electrode seat <b>46</b>, and the insulator thickness t_ialong a portion of the insulator body region <b>24</b> increases toward the center axis A and toward the insulator nose end <b>36</b> to present the electrode seat <b>46</b>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 1</figref>, the insulator thickness t_ialong the insulator body region <b>24</b> is generally constant but increases slightly at the electrode seat <b>46</b>.

p-0024The insulator nose region <b>26</b> is disposed between the insulator body region <b>24</b> and the insulator nose end <b>36</b>. The insulator <b>22</b> presents a sixth radius R₆along the insulator nose region <b>26</b> extending from the center axis A to the insulator outer surface <b>30</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The sixth radius R₆presented by the insulator nose region <b>26</b> is less than the first radius R₁presented by the insulator body region <b>24</b>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 1</figref>, the sixth radius R₆of the insulator nose region <b>26</b> tapers toward the insulator nose end <b>36</b>. The insulator thickness t_ialong the insulator nose region <b>26</b> is less than the insulator thickness t_ialong the insulator body region <b>24</b>, and the insulator thickness t_idecreases toward the insulator nose end <b>36</b>.

p-0025As shown in <figref idrefs="DRAWINGS">FIGS. 1-3</figref>, the insulator seat <b>28</b> is disposed between the insulator body region <b>24</b> and the insulator nose region <b>26</b>. The insulator seat <b>28</b> extends at an insulator seat angle α_iradially inwardly toward the center axis A and downwardly toward the insulator nose end <b>36</b>. The insulator seat angle α_iis measured relative to a plane extending perpendicular to the center axis A and intersecting the insulator seat <b>28</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The insulator thickness t_ialong the insulator seat <b>28</b> decreases from the insulator body region <b>24</b> to the insulator nose region <b>26</b>.

p-0026The insulator <b>22</b> also includes a first transition <b>48</b> extending continuously from the insulator body region <b>24</b> to the insulator seat <b>28</b>, and the first transition <b>48</b> is convex. The first radius R₁presented by the insulator body region <b>24</b> is typically constant from the insulator lower shoulder <b>44</b> to the first transition <b>48</b>. The insulator <b>22</b> also presents a fifth radius R₅at the first transition <b>48</b>, which is a spherical radius at point located along the first transition <b>48</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The spherical radius at a particular point is obtained from a sphere having a radius at that particular point. The spherical radius is the radius of the sphere in three dimensions.

p-0027A second transition <b>50</b> extends continuously from the insulator seat <b>28</b> to the insulator nose region <b>26</b>, and the second transition <b>50</b> is concave. The insulator <b>22</b> presents a second radius R₂extending from the center axis A to a point P at the intersection of the insulator outer surface <b>30</b> of the insulator seat <b>28</b> and the insulator outer surface <b>30</b> of the insulator nose region <b>26</b> adjacent the second transition <b>50</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. A fourth radius R₄is also located at the second transition <b>50</b>, and the fourth radius R₄is a spherical radius at a point located along the second transition <b>50</b>.

p-0028The insulator <b>22</b> includes an increased insulator seat angle α_i, compared to spark plug insulators of the prior art. The insulator seat angle α_iof the inventive spark plug is from 35° to 50°, whereas seat angles of the prior art are 30° or less. In one preferred embodiment, the insulator seat angle α_iis 45°, or within +/−2° of 45°.

p-0029The insulator <b>22</b> also includes an increased insulator thickness t_iaround the insulator seat <b>28</b>. The value of the fourth radius R₄is maximized, while maintaining an acceptable value for the second radius R₂. The increased insulator seat angle α_iand fourth radius R₄provides reduced tensile stress during assembly and increased bending strength when subjected to shock wave forces due to mega-knock or super-knock which occur during use of the spark plug <b>20</b> in a combustion engine.

p-0030The insulator seat angle α_iis also greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R₁−R₂)÷(R₄+R₅)]. When manufacturing the insulator <b>22</b>, the method typically includes selecting a desired insulator seat angle α_ifrom 35° to 50°, and then using the equation to determine values for R₁, R₂, R₃, R₄, and R₅that provide a boundary value less than or equal to the desired seat angle. The method typically includes adjusting at least one of the values of R₁, R₂, R₃, R₄, and R₅to obtain the desired insulator geometry. For example, the value of R₄is typically increased to a maximum value that provides the desired seat angle while maintaining an acceptable value of R₂. The insulator seat angle α_iis preferably not greater than 300%, more preferably not greater than 200%, and yet more preferably not more than 150% of the boundary value obtained by the equation.

p-0031The insulator <b>22</b> is formed of an electrically insulator <b>22</b> material, and preferably a material having a dielectric strength of 14 to 30 kV/mm, a coefficient of thermal expansion (CTE) between 2×10⁻⁶PC and 18×10⁻⁶/° C., and a relative permittivity of 2 to 12. In one embodiment, the electrically insulating material includes alumina. A coating (not shown) can optionally be applied to the insulator outer surface <b>30</b>. The coating typically includes nickel or copper.

p-0032The spark plug <b>20</b> of <figref idrefs="DRAWINGS">FIG. 1</figref> also includes a center electrode <b>52</b>, a terminal <b>54</b>, a seal <b>56</b>, a shell <b>58</b>, a pair of gaskets <b>60</b>, <b>62</b>, and a ground electrode <b>64</b>. The center electrode <b>52</b> is received in the bore of the insulator <b>22</b> and extends longitudinally along the center axis A from an electrode terminal end <b>66</b> past the insulator nose end <b>36</b> to a center electrode firing end <b>100</b>. The center electrode <b>52</b> includes a head at the electrode terminal end <b>66</b> resting on the electrode seat <b>46</b> of the insulator <b>22</b>. A terminal <b>54</b> is received in the bore of the insulator <b>22</b> and extends longitudinally along the center axis A from an energy input end <b>68</b> to an energy output end <b>70</b> spaced from electrode terminal end <b>66</b>. A seal <b>56</b> is also contained in the bore of the insulator <b>22</b> and extends continuously between the energy output end <b>70</b> of the terminal <b>54</b> and the electrode terminal end <b>66</b>. The seal <b>56</b> can be resistive or non-resistive.

p-0033The shell <b>58</b> is formed of a metal material, preferably steel, and is disposed annularly around the insulator <b>22</b>. The shell <b>58</b> extends longitudinally from a shell upper end <b>72</b> along the insulator transition region <b>40</b> and the insulator body region <b>24</b> to a shell lower end <b>74</b>. The shell <b>58</b> presents a shell inner surface <b>76</b> facing the insulator outer surface <b>30</b> and a shell outer surface <b>78</b> facing opposite the shell inner surface <b>76</b>. The shell inner surface <b>76</b> and the shell outer surface <b>78</b> each extend from the shell upper end <b>72</b> to the shell lower end <b>74</b>, and the shell inner surface <b>76</b> and the shell outer surface <b>78</b> present a shell thickness t_stherebetween. As shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, the shell <b>58</b> has a shell outer diameter D₃, which is typically 12 mm, but can alternatively be from 8 mm to 18 mm.

p-0034The shell <b>58</b> includes a shell body region <b>80</b> extending along the center axis A between the shell upper end <b>72</b> and the shell lower end <b>74</b>. The shell <b>58</b> presents a seventh radius R₇along the shell body region <b>80</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The seventh radius R₇extends from the center axis A to the shell inner surface <b>76</b>. The top of the shell <b>58</b> is bent such that the shell upper end <b>72</b> rests on the insulator upper shoulder <b>42</b>. The shell lower end <b>74</b> is disposed along the insulator nose region <b>26</b> such that the insulator nose end <b>36</b> is disposed outwardly of the shell lower end <b>74</b>.

p-0035The shell <b>58</b> includes a rib <b>82</b> adjacent the insulator seat <b>28</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 1-3</figref>. The rib <b>82</b> extends radially toward the center axis A and is disposed between the shell body region <b>80</b> and the shell lower end <b>74</b>. The shell thickness t_sis constant along the insulator body region <b>24</b> and increases adjacent the insulator seat <b>28</b> to present the rib <b>82</b>. The rib <b>82</b> includes a shell seat <b>84</b> preferably facing parallel to the insulator seat <b>28</b> and extending radially inwardly toward the center axis A and downwardly toward the shell lower end <b>74</b>. The shell seat <b>84</b> extends at a shell seat angle α_swhich is relative to a plane extending perpendicular to the center axis A and intersecting the shell seat <b>84</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The shell seat angle α_sis preferably equal to the insulator seat angle α_ior within +/−1° of the insulator seat angle α_i.

p-0036The shell seat <b>84</b> extends from the shell body region <b>80</b> to a rib inner surface <b>86</b>. The shell thickness t_sincreases gradually along the shell seat <b>84</b> to the rib inner surface <b>86</b> and is constant along the rib inner surface <b>86</b>. In the embodiment of <figref idrefs="DRAWINGS">FIG. 1</figref>, the rib inner surface <b>86</b> is disposed at the innermost point of the shell inner surface <b>76</b>. The shell <b>58</b> presents a third radius R₃at the rib inner surface <b>86</b> extending from the center axis A to the shell inner surface <b>76</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. The third radius R₃is less than the seventh radius R₇of the shell body region <b>80</b>. The rib <b>82</b> also includes a rib lower surface <b>88</b> facing toward the shell lower end <b>74</b>. The rib lower surface <b>88</b> extends radially outwardly from the rib inner surface <b>86</b> at an angle. The shell thickness t_sdecreases along the rib lower surface <b>88</b> toward the shell lower end <b>74</b>. The shell outer surface <b>78</b> includes threads along at least a portion of the shell body region <b>80</b> and adjacent the rib <b>82</b>, so that the shell <b>58</b> can be threaded into a cylinder head.

p-0037The spark plug <b>20</b> of <figref idrefs="DRAWINGS">FIG. 1</figref> includes a first gasket <b>60</b> compressed between the insulator seat <b>28</b> and the shell seat <b>84</b>, and can include a second gasket <b>62</b> compressed between the insulator upper shoulder <b>42</b> and the shell upper end <b>72</b>. The gaskets <b>60</b>, <b>62</b> are formed of a metal material, such as steel or copper.

p-0038The first gasket <b>60</b> has a gasket inner surface <b>90</b> facing generally toward the insulator <b>22</b> and a gasket outer surface <b>92</b> facing generally toward the shell <b>58</b>. The gasket inner surface <b>90</b> and the gasket outer surface <b>92</b> both extend from a gasket top surface <b>94</b> to a gasket bottom surface <b>96</b>. A lubricant (not shown) may be applied to the gasket during assembly of the spark plug <b>20</b>. The gasket top surface <b>94</b> and gasket bottom surface <b>96</b> present a friction coefficient, which depends on the material used to form the gasket and whether lubricant is applied to the gasket. Reducing friction at this gasket interface, for example by adding a lubricant or by coating the gasket in a low-friction material, leads to a reduction in the tensile stress created by the assembly process; but only for lower seat angles. The friction-reducing coating is preferably located between the gasket and the shell. As the seat angle increases a point is reached where the gasket begins to slide on the shell and the tensile stress increases sharply due to deformation of the insulator seat <b>28</b>. If the friction coefficient is less than or equal to 0.15, then the insulator seat angle α_iis preferably from 35° to 45°. If the friction coefficient is greater than 0.15, then the insulator seat angle α_ican be up to 50°.

p-0039The first gasket <b>60</b> presents an outer gasket thickness t_g1extending from the gasket top surface <b>94</b> to the gasket bottom surface <b>96</b> at the gasket outer surface <b>92</b>. The first gasket <b>60</b> also presents an inner gasket thickness t_g2extending from the gasket top surface <b>94</b> to the gasket bottom surface <b>96</b> at the gasket inner surface <b>90</b>. As shown in <figref idrefs="DRAWINGS">FIG. 2A</figref>, the outer gasket thickness t_g1is greater than the inner gasket thickness t_g2. The inner gasket thickness t_g2is preferably greater than or equal to 70% of the outer gasket thickness t_g1.

p-0040The ground electrode <b>64</b> is attached to the shell <b>58</b>, as shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, and extends from the shell lower end <b>74</b> to a ground electrode firing end <b>102</b>. The ground electrode <b>64</b> extends parallel to the center axis A and then curves toward the center axis A. The ground electrode <b>64</b> presents a ground spark surface <b>98</b> facing parallel to and spaced from the center electrode firing end <b>100</b> such that the center electrode firing end <b>100</b> and the ground spark surface <b>98</b> present a spark gap therebetween.

p-0041Another aspect of the invention provides a method of manufacturing the spark plug <b>20</b> including an insulator <b>22</b> with the insulator seat angle α_ibeing from 35° to 50° and the insulator seat angle α_ibeing greater than or equal to a boundary value provided by the equation: 90°−a cos [1−(R₁−R₂)÷(R₄+R₅)].

p-0042The method first comprises selecting a value for the insulator seat angle α_i(α_i) between 35° to 50°. The method next includes obtaining values for R₁, R₂, R₄, and R₅. The values can be calculated using various different methods. The value of R₄is preferably maximized while maintaining an acceptable value of R₂. Once the values of R₁, R₂, R₄, and R₅are obtained, the method includes determining whether the selected insulator seat angle α_iis greater than or equal to the boundary value provided by the equation. If the selected insulator seat angle α_iis greater than or equal to the boundary value, then the method can include forming the insulator <b>22</b> with the selected insulator seat angle α_iand obtained values of R₁, R₂, R₄, and R₅.

p-0043If the selected insulator seat angle α_iis less than the boundary value, then the method includes adjusting at least one of the values of R₁, R₂, R₄, and R₅so that the boundary value is greater than or equal to the selected insulator seat angle α_i.

p-0044Alternatively, even if the boundary value is greater than or equal to the selected insulator seat angle α_i, the method can include adjusting at least one of the values of R₁, R₂, R₄, and R₅so that the boundary value is closer to the selected insulator seat angle α_i. For example, the method could include increasing the selected value of R₄and decreasing R₂while maintaining the insulator seat angle α_igreater than or equal to the boundary value. The selected insulator seat angle α_iis preferably not greater than 300% of the boundary value, more preferably not greater than 200% of the boundary value, and yet more preferably not greater than 150% of the boundary value.

p-0045The method also includes obtaining a value for the third radius R₃, which is at the rib inner surface <b>86</b> of the shell <b>58</b> and extends from the center axis A to the shell inner surface <b>76</b>. The method next includes determining whether the selected value for R₃allows the selected insulator seat angle α_ito be greater than or equal to the boundary value. If the selected insulator seat angle α_iis less than the boundary value, then the method includes adjusting at least one of the values of R₁, R₂, R₃, R₄, and R₅.

p-0046Once the geometry of the insulator <b>22</b> and the shell <b>58</b> is determined, the method next includes compressing the first gasket <b>60</b> between the insulator seat <b>28</b> and the shell seat <b>84</b>. The outer gasket thickness t_g1is preferably greater than the inner gasket thickness t_g2after the step of compressing the first gasket <b>60</b>.

EXPERIMENT

p-0047Spark plugs of this invention are calculated by Finite Element Analysis (FEA) to have a lower tensile stress due to plug assembly which leads directly to reduced stress in bending. The geometry changes described here also lead to an additional reduction in stress due to bending loads, due to better distribution of load. An experiment was conducted to compare the bending strength during use of the inventive spark plug <b>20</b> having a shell outer diameter D₃of 12 mm and an insulator seat angle α_iof 45° to a comparative spark plug having a shell outer diameter of 12 mm and insulator seat angle of 30°. The insulator <b>22</b> of the first inventive embodiment, shown in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>; the insulator <b>22</b> of the second inventive embodiment, shown in <figref idrefs="DRAWINGS">FIG. 3</figref>; and the insulator of the comparative spark plug, shown in <figref idrefs="DRAWINGS">FIG. 4</figref>, were each tested. Table 1 provides R₁-R₅for each of the spark plugs. Table 1 also provides the boundary value for each of the spark plugs, and the insulator seat angle αs a percentage of the boundary value.

p-0048<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="5"><colspec colname="offset" colwidth="14pt" align="left" /><colspec colname="1" colwidth="35pt" align="center" /><colspec colname="2" colwidth="63pt" align="center" /><colspec colname="3" colwidth="49pt" align="center" /><colspec colname="4" colwidth="56pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="4" rowsep="1">TABLE 1</entry></row><row><entry /><entry namest="offset" nameend="4" align="center" rowsep="1" /></row><row><entry /><entry /><entry>First</entry><entry>Second</entry><entry>Comparative</entry></row><row><entry /><entry /><entry>Embodiment</entry><entry>Embodiment</entry><entry>Spark Plug</entry></row><row><entry /><entry>Dimension</entry><entry>(FIGS. 1 and 2)</entry><entry>(FIG. 3)</entry><entry>(FIG. 4)</entry></row><row><entry /><entry namest="offset" nameend="4" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>α</entry><entry>45°</entry><entry>45°</entry><entry>30°</entry></row><row><entry /><entry>R₁</entry><entry>0.145″/</entry><entry>0.145″/</entry><entry>0.145″/</entry></row><row><entry /><entry /><entry>3.683 mm</entry><entry>3.683 mm</entry><entry>3.683 mm</entry></row><row><entry /><entry>R₂</entry><entry>0.105″/</entry><entry>0.095″/</entry><entry>0.100″/</entry></row><row><entry /><entry /><entry>2.667 mm</entry><entry>2.431 mm</entry><entry>2.540 mm</entry></row><row><entry /><entry>R₃</entry><entry>0.121″/</entry><entry>0.121″/</entry><entry>0.121″/</entry></row><row><entry /><entry /><entry>3.073 mm</entry><entry>3.073 mm</entry><entry>3.073 mm</entry></row><row><entry /><entry>R₄</entry><entry>0.080″/</entry><entry>0.120″/</entry><entry>0.030″/</entry></row><row><entry /><entry /><entry>2.032 mm</entry><entry>2.048 mm</entry><entry>0.762 mm</entry></row><row><entry /><entry>R₅</entry><entry>0.020″/</entry><entry>0.020″/</entry><entry>0.020″/</entry></row><row><entry /><entry /><entry>0.508 mm</entry><entry>0.508 mm</entry><entry>0.508 mm</entry></row><row><entry /><entry>Boundary</entry><entry>36.87</entry><entry>40.00</entry><entry>5.74</entry></row><row><entry /><entry>α as % of</entry><entry>122%</entry><entry>112%</entry><entry>523%</entry></row><row><entry /><entry>Boundary</entry></row><row><entry /><entry namest="offset" nameend="4" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

p-0049The FEA results indicate the average tensile stress during assembly of the inventive spark plug <b>20</b> according to the first embodiment and the second embodiment is less than the average tensile stress during assembly of the comparative spark plug and indicate an improvement in bending strength. Table 2 and <figref idrefs="DRAWINGS">FIG. 5</figref> provides the bending strength test results, and illustrate the average bending strength of the inventive spark plug <b>20</b> according to the first embodiment and the second embodiment is greater than the average bending strength of the comparative spark plug.

p-0050<tables id="TABLE-US-00002" num="00002"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="84pt" align="left" /><colspec colname="1" colwidth="49pt" align="center" /><colspec colname="2" colwidth="42pt" align="center" /><colspec colname="3" colwidth="42pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="3" rowsep="1">TABLE 2</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row><row><entry /><entry>First</entry><entry>Second</entry><entry>Comparative</entry></row><row><entry /><entry>Embodiment</entry><entry>Embodiment</entry><entry>Spark Plug</entry></row><row><entry /><entry>(FIGS. 1 and 2)</entry><entry>(FIG. 3)</entry><entry>(FIG. 4)</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="49pt" align="center" /><colspec colname="3" colwidth="42pt" align="center" /><colspec colname="4" colwidth="42pt" align="center" /><tbody valign="top"><row><entry>Average bending strength</entry><entry>901N</entry><entry>728N</entry><entry>609N</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

p-0051Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.

p-0052<tables id="TABLE-US-00003" num="00003"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>ELEMENT LIST</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="2"><colspec colname="1" colwidth="105pt" align="center" /><colspec colname="2" colwidth="112pt" align="left" /><tbody valign="top"><row><entry>Element Symbol</entry><entry>Element Name</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row><row><entry>A</entry><entry>center axis</entry></row><row><entry>D₁</entry><entry>insulator inner diameter</entry></row><row><entry>D₂</entry><entry>insulator outer diameter</entry></row><row><entry>P</entry><entry>point</entry></row><row><entry>20</entry><entry>spark plug</entry></row><row><entry>22</entry><entry>insulator</entry></row><row><entry>24</entry><entry>insulator body region</entry></row><row><entry>26</entry><entry>insulator nose region</entry></row><row><entry>28</entry><entry>insulator seat</entry></row><row><entry>30</entry><entry>insulator outer surface</entry></row><row><entry>32</entry><entry>insulator inner surface</entry></row><row><entry>34</entry><entry>insulator upper end</entry></row><row><entry>36</entry><entry>insulator nose end</entry></row><row><entry>38</entry><entry>insulator terminal region</entry></row><row><entry>40</entry><entry>insulator transition region</entry></row><row><entry>42</entry><entry>insulator upper shoulder</entry></row><row><entry>44</entry><entry>insulator lower shoulder</entry></row><row><entry>46</entry><entry>electrode seat</entry></row><row><entry>48</entry><entry>first transition</entry></row><row><entry>50</entry><entry>second transition</entry></row><row><entry>52</entry><entry>center electrode</entry></row><row><entry>54</entry><entry>terminal</entry></row><row><entry>56</entry><entry>seal</entry></row><row><entry>58</entry><entry>shell</entry></row><row><entry>60</entry><entry>first gasket</entry></row><row><entry>62</entry><entry>second gasket</entry></row><row><entry>64</entry><entry>ground electrode</entry></row><row><entry>66</entry><entry>electrode terminal end</entry></row><row><entry>68</entry><entry>energy input end</entry></row><row><entry>70</entry><entry>energy output end</entry></row><row><entry>72</entry><entry>shell upper end</entry></row><row><entry>74</entry><entry>shell lower end</entry></row><row><entry>76</entry><entry>shell inner surface</entry></row><row><entry>78</entry><entry>shell outer surface</entry></row><row><entry>80</entry><entry>shell body region</entry></row><row><entry>82</entry><entry>rib</entry></row><row><entry>84</entry><entry>shell seat</entry></row><row><entry>86</entry><entry>rib inner surface</entry></row><row><entry>88</entry><entry>rib lower surface</entry></row><row><entry>90</entry><entry>gasket inner surface</entry></row><row><entry>92</entry><entry>gasket outer surface</entry></row><row><entry>94</entry><entry>gasket top surface</entry></row><row><entry>96</entry><entry>gasket bottom surface</entry></row><row><entry>98</entry><entry>ground spark surface</entry></row><row><entry>100</entry><entry>center electrode firing end</entry></row><row><entry>102</entry><entry>ground electrode firing end</entry></row><row><entry>α_i</entry><entry>insulator seat angle</entry></row><row><entry>α_s</entry><entry>shell seat angle</entry></row><row><entry>R₁</entry><entry>first radius</entry></row><row><entry>R₂</entry><entry>second radius</entry></row><row><entry>R₃</entry><entry>third radius</entry></row><row><entry>R₄</entry><entry>fourth radius</entry></row><row><entry>R₅</entry><entry>fifth radius</entry></row><row><entry>R₆</entry><entry>sixth radius</entry></row><row><entry>R₇</entry><entry>seventh radius</entry></row><row><entry>t_g1</entry><entry>outer gasket thickness</entry></row><row><entry>t_g2</entry><entry>inner gasket thickness</entry></row><row><entry>t_i</entry><entry>insulator thickness</entry></row><row><entry>t_s</entry><entry>shell thickness</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>